Regulated power supplies typically incorporate a means to sense the deviation of the output voltage from the desired value, the output of which controls a means of adjusting the output voltage to restore it to the desired value. This feedback control loop typically does operate instantaneously, especially in the face of sudden changes in the current demanded by the load being supported. To alleviate this, a capacitor, which can provide current to the load during the brief period before the feedback loop can respond, is usually placed at the supply output.
This combination, typically does not maintain the supply output completely stable in the face of load current changes. In particular, the presence of the capacitor, while improving performance by briefly providing current to the load during transients, also tends to degrade performance by slowing down the response of the control loop. This problem can be partly mitigated with sophisticated design of the output voltage control loop. In particular, using the output voltage error to control the current delivered into the output capacitor (an inner current control loop, creating a voltage-controlled current source) allows a very fast voltage control loop while still maintaining stability.